As used herein, the following abbreviations shall have the following meanings:                3GPP Third Generation Partnership Project        ACM Address Complete Message        BSS Base Station Subsystem        CAMEL Customized Applications for Mobile networks Enhanced Logic        CDMA Code Division Multiple Access        CS Circuit Switched        DTM/PSHO Dual Transfer Mode/Packet Switched Handover        eNodeB Evolved Node B        EPS Evolved Packet System        E-UTRAN Evolved-UTRAN        GAN Generic Access Network        GANC GAN Controller        GBR Guaranteed Bit Rate        GERAN GSM EDGE Radio Access Network        GSM Global System for Mobile Communications        GUTI Temporary UE Identity        GW Gateway        HLR Home Location Register        HO Handover        HSS Home Subscriber Server        IMS IP Multimedia Subsystem        IAM Initial Address Message        IMSI International Mobile Subscriber Identity        IP Internet Protocol        ISDN Integrated Services Digital Network        ISUP ISDN User Part        LTE Long Term Evolution        MM Mobility Management        MME Mobility Management Entity        MMTel Multimedia Telephony        MSC Mobile Switching Center        MSISDN Mobile Station International Subscriber Directory Number        Non-GBR Non-Guaranteed Bit Rate        PS Packet Switched        QCI QoS Class Identifier        RAN Radio Access Network        RANAP Radio Access Network Application Part        S-GW Signaling Gateway        SDP Session Description Protocol        SES Send End Signal        SGSN Serving GPRS Support Node        SRVCC Single Radio Voice Call Continuity        STN-SR Station Transfer Number for SRVCC        TAU Tracking Area Update        UE User equipment        UMTS Universal Mobile Telecommunications System        UTRAN UMTS Radio Access Network        VCC Voice Call Continuity        VDN VCC Directory Number        VoIP Voice over Internet Protocol        VPLMN Visited Public Land Mobile Network        WCDMA Wideband CDMA        
The 3rd Generation Partnership Project (3GPP) is a collaboration between groups of telecommunications associations, to make a globally applicable third generation (3G) mobile phone system specification within the scope of the International Mobile Telecommunications-2000 project of the International Telecommunication Union (ITU). 3GPP specifications are based on evolved Global System for Mobile Communications (GSM) specifications. 3GPP standardization encompasses Radio, Core Network and Service architecture. The project was established in December 1998.
System Architecture Evolution (SAE) is the core network architecture of 3GPP's future Long Term Evolution (LTE) wireless communication standard. SAE is the evolution of the GPRS Core Network, but with a simplified architecture, all IP Network, support for higher throughput and lower latency radio access networks (RANs) and support for, and mobility between, multiple heterogeneous RANs, including legacy systems as GPRS, but also non-3GPP systems such as WiMAX.
The main component of the EPC architecture is the Evolved Packet Core network (EPC). The EPC will be the equivalent of the GPRS networks via the Mobility Management Entity (MME), Serving Gateway and PDN Gateway, and the Home Subscriber Server (HSS) subcomponents. The MME is the key control-node for the LTE access-network. The Serving Gateway routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies. The PDN Gateway provides connectivity from the UE to external packet data networks by being the point of exit and entry of traffic for the UE. The HSS is the Home Subscriber Server holding data for the subscribers.
The 3GPP Specification provides different methods to support a voice service via EPS. One is IMS MMTel which may be required to use SRVCC if there is no VoIP over packet switched (PS) network radio bearers in the entire wide area network. SRVCC supports IMS Voice with a mechanism to move the GSM, WCDMA, or cdma2000 1xRTT access and support voice service using a CS network bearer rather than a packet bearer which is the primary choice for an IMS based voice service.
Another method to support a voice service via EPS is CS fallback wherein the user has access to the voice service while being connected to the LTE access, but the voice service is provided by an access that has (native) support for a CS voice service, i.e. GSM, WCDMA, or cdma2000 1xRTT. Furthermore, there are industry initiatives to improve CS fallback by re-using some of the SRVCC mechanisms. However, re-using standardized methods provided by 3GPP may create conflicts among the different proposals.
3GPP TS 43.318 entitled, “Generic Access Network (GAN); Stage 2 (Release 8)” defines the stage 2 service description for a Generic Access Network (GAN). It describes the GAN system concepts, documents the reference architecture, functional entities, network interfaces, and high-level procedures. Two modes of operation are described: GAN A/Gb and GAN lu. The GAN A/Gb mode is an extension of GSM/GPRS mobile services that supports tunneling of Non Access Stratum (NAS) protocols between the MS and the core network over an IP network and the A and Gb interfaces to the MSC and SGSN, respectively. The GAN lu mode does the same in an extension of UMTS mobile services using the lu-cs and lu-ps interfaces to the MSC and SGSN, respectively;
One industry proposal envisions tunneling CS service over IP using the GAN and SRVCC standard.
The architecture 100 in which CS over LTE using GANC is performed is seen in FIG. 1. To support hand-over (HO), the SRVCC mechanisms are re-used such that the UE 101 simulates the SRVCC capability.
To support CS over LTE using GANC, the following E-UTRAN attach procedure, in addition to 3GPPP TS 23.401, is required. CS over LTE using GANC will use the SRVCC additions as follows:
In CS over LTE, the UE 101 includes the SRVCC capability indication as part of the UE network capability in its Attach Request message. A SRVCC capability indicator is used instead of a CS over LTE specific indicator, as no standard exists for CS over LTE using GANC. The MME 104 stores this information for what it believes to be used for SRVCC operation, but in fact is used for CS over LTE.
The eNodeB of the E-UTRAN will receive a UE SRVCC capability indication as part of UE radio capability or from the MME 104 depending on how the standard evolves.
If the subscriber is allowed to have SRVCC in the VPLMN then the HSS includes SRVCC STN-SR and MSISDN in the Insert Subscriber Data message to the MME in normal cases. However because CS over LTE is a proprietary solution, roaming outside home network with SRVCC indication will be disallowed, thus only 2G and 3G will be allowed for roaming users.
To make the process work, the MME and eNodeB must accept that the UE is capable of the SRVCC solution. The eNodeB would signal that a certain HO request sent to the MME is an SRVCC HO or that the target cell capability does not support IMS Voice, or VoIP in general. The foregoing option has not yet been determined by 3GPP. The MME would use the information from the eNodeB, together with the terminal capability and subscription information downloaded from HSS during attach, to determine whether the SRVCC “bearer splitting function” and the Sv interface, both defined for SRVCC, are to be used to perform a PS network to CS network HO.
In 3GPP, the Sv interface in the SRVCC is between the MME and the MSC and in the industry initiative the Sv interface 105 is re-used as an interface between the MME and a GANC as seen in FIG. 1.
FIG. 2 illustrates the HO procedure 200 for CS over LTE GANC that re-uses SRVCC procedures for a handover between CS over LTE to GSM:
Step 201. Based on UE measurement reports, the source E-UTRAN determines whether to trigger a handover to GERAN.
Step 202. The source E-UTRAN sends a Handover Required (Source to Target Transparent Container) message to the source MME. The E-UTRAN also indicates to the MME that this is a SRVCC HO operation. A SRVCC indicator is used as a CS over LTE indicator does not exist. Although the MME and eNodeB interpret this as SRVCC, it is a CS over LTE GANC solution.
Step 203. Based on the QCI associated with the voice bearer (QCI 1 or similar depending on the determination of the 3GPP) and the SRVCC HO indication, the source MME splits the voice bearer from the non voice bearers and initiates the PS network handover procedure for the voice bearer by sending a Forward Relocation Request (VDN/MSISDN), Source to Target Transparent Container, MM Context message to the GANC. The MME receives the MSISDN from the HSS as part of the subscription profile downloaded during the E-UTRAN attach procedure. The MM Context contains security related information. The security related information will not be required if the GANC has that information. Note that VDN and MISDN may not be used by the GANC, as it is part of SRVCC. The GANC discards the information.
Step 204. The GANC interworks the PS network HO request with a RANAP Relocation Required over the Iu interface to the MSC. GANC is regarded as the source RAN by the MSC. The CS Security key may need to be derived.
Step 205. The target MSC performs resource allocation with the target BSS by exchanging Handover Request/Acknowledge messages. If the Inter-MSC HO must be performed, normal inter-MSC procedures will be exchanged between the source MSC and target MSC.
Step 206. When resources have been allocated in BSS, the MSC sends a RANAP relocation command to the GANC.
Step 207. The GANC interworks the RANAP relocation command to a Forward Relocation Response (Target to Source Transparent Container) message which is sent to the source MME. The source MME acknowledges that at the end of the CS network to PS network HO the non-GBR bearers should be preserved.
Step 208a. The source MME sends a Handover Command (Target to Source Transparent Container) message to the source E-UTRAN. The message includes information about the voice component only.
Step 208b. In parallel to the previous step, the source MME exchanges Update Bearer Request/response messages with the Serving GW. The S1-U bearers are released for all EPS bearers and the voice bearer is deactivated. The non-GBR bearers are preserved. The MME is aware that a PS network to CS network HO has occurred due to SRVCC, so upon receipt of downlink data notification from the S-GW the MME shall not page the UE.
Step 209. Source E-UTRAN sends a HO from E-UTRAN Command message to the UE. There may be an indication to UE to inform PS network to CS network HO.
Step 210. HO Detection at the target BSS.
Step 211. The target BSS sends a HO Complete message to the target MSC.
Step 212. The MSC sends a RANAP release command to GANC.
Step 213. The RANAP is release complete.
Step 214. The GANC sends a Forward Relocation Complete message to the source MME, informing it that the UE has arrived on the target side. The source MME acknowledges the information by sending a Forward Relocation Complete Acknowledge message to the GANC.
Step 215. The call has moved over to the CS network In SRVCC, the MME selects the MSC based on the target cell identity. Consequently, the MME is unable to determine which node, an MSC or GANC, it should signal to use the Sv interface procedures if both are present in the network.
Hence, two problems occur: (1) it is not possible to have both methods in the network, because there is only one terminal capability (“SRVCC capable”) used for both methods and (2) a network cannot support SRVCC or the industry initiative solution to incoming roaming users, since the interpretation of the terminal capability (“SRVCC capable”) is ambiguous.
FIGS. 3 and 4 provide selected parts from 3GPP TS 23.216. FIG. 3 illustrates the SRVCC architecture 300 for E-UTRAN to 3GPP UTRAN/GERAN. FIG. 4 illustrates the steps 400 for SRVCC from E-UTRAN to GERAN without DTM/PSHO support. As seen therein:
Step 401. Based on UE measurement reports, the source E-UTRAN determines to trigger an SRVCC HO to GERAN.
Step 402. The source E-UTRAN sends Handover Required (Target ID, Source to Target Transparent Container) message to the source MME. The E-UTRAN also indicates to the MME that this is a SRVCC HO operation.
Step 403. Based on the QCI associated with the voice bearer (QCI 1) and the SRVCC handover indication, the source MME splits the voice bearer from the non voice bearers and initiates the PS network to CS network HO procedure for the voice bearer only towards MSC Server.
Step 404. The MME sends a Forward Relocation Request (STN-SR, MSISDN, Source to Target Transparent Container, MM Context) message to the MSC server. The MSC server is selected based on the Target ID received in the Handover Required message. The MME receives the STN-SR and MSISDN from the HSS as part of the subscription profile downloaded during the E-UTRAN attach procedure. The MM Context contains security related information. The CS security key is derived by the MME from the E-UTRAN/EPS domain key as defined in TS 33.401. The CS security key is sent in the MM Context.
Step 405. The MSC Server interworks the PS network to CS network handover request with a CS inter-MSC handover request by sending a Prepare Handover Request message to the target MSC.
Step 406. The target MSC performs resource allocation with the target BSS by exchanging Handover Request/Acknowledge messages.
Step 407. The target MSC sends a Prepare Handover Response message to the MSC Server.
Step 408. Establishing a circuit connection between the target MSC and the MGW associated with the MSC server e.g., using ISUP IAM and ACM messages.
Step 409. The MSC server initiates the Session Transfer by using the STN-SR, that is, by sending an ISUP IAM (STN-SR) message towards the IMS. Standard IMS Service Continuity procedures are applied for execution of the Session Transfer, as discussed in TS 23.292 and TS 23.237. During the execution of the Session Transfer procedure, the remote end is updated with the SDP of the CS network access leg. The downlink flow of VoIP packets is switched towards the CS network access leg at this point. Note, however, that if the MSC server is using an ISUP interface, then the initiation of the session transfer may fail if the subscriber profile including CAMEL triggers is not available prior HO (see clause 7.3.2.1 in TS 23.292 [13]).
Step 410. The MSC server sends a Forward Relocation Response (Target to Source Transparent Container) message to the source MME. The source MME preserves the non-GBR bearers at the end of the PS network to CS network HO.
Step 411. The source MME sends a HO Command (Target to Source Transparent Container) message to the source E-UTRAN. The message includes information about the voice component only.
Step 412. The source E-UTRAN sends a HO from E-UTRAN Command message to the UE.
Step 413. Handover Detection at the target BSS.
Step 414. The UE starts the Suspend procedure specified in TS 23.060 [10], clause 16.2.1.1.2. This triggers the Target SGSN to send a Suspend Request message to the Source MME. The MME returns a Suspend Response to the Target SGSN, which contains the MM and PDP contexts of the UE. The MME also starts the preservation of non-GBR bearers and the deactivation of the voice bearer.
Step 415. The target BSS sends a HO Complete message to the target MSC.
Step 416. The target MSC sends an SES (HO Complete) message to the MSC server.
Step 417. Completion of the establishment of the circuit connection between the MSC and the MGW associated with the MSC server is completed, e.g., with the target MSC sending ISUP Answer message to the MSC server.
Step 418. The MSC server sends a Forward Relocation Complete message to the source MME, informing it that the UE has arrived on the target side. The source MME acknowledges the information by sending a Forward Relocation Complete Acknowledge message to the MSC server.
Step 419. The MSC server may perform a MAP Update Location to the HSS/HLR if needed. This may be needed for MSC Server to receive GSM Supplementary Service information and routing of mobile terminating calls properly in certain configuration. Note, however, that this Update Location is not initiated by the UE.
After the CS voice call is terminated and if the UE is still in GERAN, then (as specified in TS 23.060) the UE shall resume PS network services by sending a Routing Area Update Request message to the SGSN. The Update Type depends on the mode of operation of the GERAN network, e.g., in mode I, a Combined RA/LA Update is used and in mode II or III, a Routing Area Update is used.